Branch support for mirror libraries

[cliffano]

Currently mirror libraries only support versioned releases, but we also need to add support for branch download.

Need to refactor AocMirrorGithubLibrary to do the following:

• check if branch exists, then download branch artifact at https://github.com/${user}/${repo}/archive/${branch}.tar.gz , and then repackage this artifact to remove the ${repo}/${branch} subdir, this way it's consistent with the released version which doesn't have such subdir
• check if version exists, then download released version at https://github.com/${user}/${repo}/releases/download/${version}/${repo}-${version}.tar.gz
• otherwise, log error message